PSI - Issue 2_B

Radomila Konečná et al. / Procedia Structural Integrity 2 (2016) 2381 – 2388 Radomila Kone č na et al./ Structural Integrity Procedia 00 (2016) 000–000

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section (i.e. 5 x 5 mm 2 ) is small. The trend in a linear-log plot is well behaved with fatigue lives of the specimens with the C orientation that are considerably different and shorter than the other two specimen orientations (i.e. A and B of Fig. 2). The presence of an anisotropic fatigue behavior of SLM IN 718 is therefore demonstrated and quantified. The other two orientations show a similar behavior with a slight higher performance of orientation A. The present directional fatigue data agree with previous findings obtained with the same test method in DMLS Ti 6Al-4V by Nicoletto et al. (2015).

Fig. 5. Fatigue life of SLM IN 718 specimens in dependence on their orientation with respect to build orientation (see Fig. 2).

Residual stresses that develop during the SLM process can seriously influence the fatigue properties, both in terms of crack initiation and crack propagation. The present specimens were heat treated to remove the residual stress fields. The X-ray measurement of residual stresses on the specimen surfaces where the fatigue cracks initiated was performed to ensure that the stress relieving procedure was successful. Stresses in two directions were measured, namely in the longitudinal direction coinciding with the longer axis of the specimen and in the transversal direction. The measurement led to the conclusion that only very weak (i.e. not exceeding 40 MPa) compressive residual stresses are present in the surface layer. 3.3. Fatigue crack initiation from as-produced surfaces Fig. 6 allows the discussion of two correlated aspects occurring in the case of high cycle fatigue testing: surface quality (i.e. surface roughness) and fatigue crack initiation mechanisms. Figs. 6a, 6c and 6e show the outline in white of the broken specimens, one for each orientation (fracture surface on the right) and identification of the top surface where the roughness measurement was performed (on the left). Tab. 2 provides the roughness data that show that the specimen orientation A is characterized by the minimum roughness and specimen orientation C by the maximum roughness. Apparently, surface roughness and fatigue strength as shown in Fig. 5 are reasonably correlated.

Table 2. Average surface roughness parameters vs specimen orientation. A B C Ra [µm] 6.49 10.67 15.56 Rq [µm] 7.58 12.97 19.63 Rz [µm] 29.67 56.96 84.43